1. Field of the invention
The present invention relates to a distance measuring apparatus for optically measuring a distance, and, more specifically, to a distance measuring apparatus based on a triangulation system and utilizing a two-dimensional image obtained by an image-pickup apparatus.
2. Description of the Prior Art
Generally, in a distance measuring apparatus based on a triangulation system, which utilizes a two-dimensional image obtained by an image-pickup apparatus, it is essential to adequately determine the level of exposure to which an image-pickup apparatus is subjected for the purpose of accurately measuring a distance to an object, and the level of exposure is determined by an ordinary exposure deciding method for an image pickup apparatus.
FIG. 1 illustrates an exposure control apparatus of a conventional image-pickup apparatus comprising an optical system lens 101, an iris 102, an image sensor 103, a mean value processing circuit 104 and a comparator 105. Image sensor 103 picks up 30 images per second in the case of, for example, a motion picture. Mean value processing circuit 104 obtains a mean value of image signals from image sensor 103. Comparator 105 compares this mean value with a preset reference signal to obtain a deviation which controls an amount of exposure by adjusting iris 102. A mean value of image signals of one picture may be used, but a mean value of image signals of a plurality of successive pictures is usually used.
FIG. 2 illustrates the structure of another conventional exposure control apparatus. This apparatus is called a peak measuring system which employs a system for deciding a level of exposure from the image signal representing the brightest part of an image.
The control apparatus shown includes a peak hold circuit 106. Image signals from image sensor 103 are inputted to peak hold circuit 106 and the image signal representing the brightest part of an image is held for each picture. Comparator 105 compares the held image signal with a reference signal to obtain a deviation which controls iris 102, thereby controlling an amount of exposure.
Also, a system for deciding an amount of exposure from the brightness of a limited region in the periphery of a picture center and a system for sampling several representative points in a picture to decide the level of exposure from a mean value of the brightness of the sampled points have been proposed.
Conventional distance measuring apparatuses control, as explained above, amounts of exposure of image-pickup apparatuses. However, since an image of a distance measurement object is not always picked up in a clear contrast at an adequate exposure level, an error may be generated in a distance calculation, the resulting distance calculation may not be used for distance determination, and, in a tracking-type distance measuring apparatus, tracking may be impossible or an erroneous object may be tracked.